Ethylene oxide, an important basic chemical, is prepared industrially by oxidation of ethylene by means of oxygen in the presence of silver-comprising catalysts. For this purpose, use is usually made of supported catalysts to which the catalytically active, metallic silver has been applied by means of a suitable method.
The preparation of ethylene oxide is usually carried out using a supported silver catalyst (Ag on Al2O3) which has a silver concentration of 15-30% by weight of silver. The catalyst can comprise further active metals in addition to silver.
Processes which use structured catalyst beds which have gradients of, for example, promoters and comprise catalyst layers of differing activity or selectivity are known from the prior art.
JP-A 56005471 describes, for example, a process for preparing ethylene oxide in which alkali metal-comprising silver catalysts are used which are loaded with different amounts of alkali metal along the length of the catalyst bed so that an alkali metal concentration gradient is present along the bed. At the upper end of the catalyst bed, i.e. at the point at which the reaction gas enters, the alkali metal content of the silver catalyst should, according to this document, be lower than at the other end of the catalyst bed, i.e. where the reaction gas leaves the reactor. Since alkali metal doping of the silver catalyst results in a decrease in the activity and an increase in the selectivity of the catalyst for the formation of ethylene oxide, the reaction gas mixture comes into contact with increasingly selective and decreasingly active silver catalyst on passing through the reactor.
EP 0 557 833 A1 discloses a combination of high-selectivity and high-activity catalysts in a structured catalyst bed. A description is given of a process for preparing ethylene oxide by oxidation of ethylene by means of oxygen in the presence of silver- and promoter-comprising catalysts, with at least two silver catalysts of differing selectivity and activity being used in a combined catalyst bed.
EP 0 428 845 A1 discloses a catalyst which has a silver gradient within a single shaped catalyst body.
WO 2004/078711 discloses a process for preparing ethylene oxide, which is operated at reduced CO2 concentration. High-selectivity and high-activity structured catalyst beds are used.
In the scientific publication “Oxidation of ethylene to ethylene oxide: catalyst deactivation in an industrial run”, Montrasi et al., Applied Catalysis (1983), 5 (3), 359-369, describe a study on the deactivation of industrial EO catalysts along the reactor tube.
In the processes known from the prior art, the activity of the catalyst continually decreases over the entire time of operation of the catalyst in production operation. After an optimization phase, the catalyst reaches its maximum/optimum selectivity which then decreases again during further operation. To maintain a particular production output (work rate), the temperature is therefore usually increased during production operation. A disadvantage of this mode of operation is the continual temperature increase which is necessary for maintaining the production output in order to compensate for the continual decrease in the selectivity of the catalyst after the selectivity maximum has been reached.
An objective of catalyst research is to keep the operating temperature of the catalyst, which is an influencing factor on the deactivation, over a period as long as possible so low that the catalyst achieves the highest possible selectivities over a period of time as long as possible.